Hi Sajitha,
Greece and Turkey are great places, you'll love it there. As to your specific questions:
The cheapest time of year to go is during the Greek winter (Nov-Feb), this is outside of normal tourist times, so your bound to get some cheaper deals. However, I would say that Greece, though part of the EU, is relatively cheap compared to other places like Switzerland or France. Greece is beautiful during the summer, and the beach is much more fun when it's warm. Im not sure if the price difference would outweigh the loss in experience you might have of Greece in the summer.
Mainland Greece is alright if you like all the history stuff, temples, statues, that sort of thing, but it is nothing you can't see on the Discovery channel. I referred the nightlife and beach scene. Most everyone on the mainland lives in Athens, which is a nice city, but the islands are the real draw to Greece. Once in Athens, I'd say you will need 3 days to see everything, and feel confidant about leaving to the islands.
Turkey is awesome. Coming from the US, I was captivated by a culture that is dominated by a religion other than christianity. Istanbul is beautiful, cheap, and has an amazing nightlife. I personally am trying to go back and live there for a bit in the future.
Personally, I would stay away from cruises in Greece, and they are pointless in Turkey. Your going to want to spend more or less time on each island, so go with the flow and try not to plan to far ahead. Once in Athens, look up Argonaut travel, they are great at helping to book ferries, they'll set you up with cheap accommodations, and they are really friendly. If you don't think your budget can sustain hotel accommodations, try using Athens Backpackers (a great hostel in a great location) and use Hostels.com to book hostels on other islands.
The best advice I got when it comes to ferries is to purchase the superfast ferry to get to your destination, but the regular ferry when you are having to meet any sort of time constraint, such as a flight out of Athens. The waters can get choppy at times, so they cancel superfast ferries often because of their small size. Superfast ferries get you to your destination a hell of alot faster than regular ferries. Regular ferries are mammoths, so they rarely get canceled, but as i said earlier, they are extremely slow.
Island tours are a breeze, and they are much cheaper than you'd think. You'll see different companies as you walk around the towns. They all have pictures and prices clearly posted in the windows as displays.
I can give you more info on some specific islands, but I don't know what kind of things your looking to do. Are you a history buff or a wanderer? A sit inside with a book or party till 6am? Do you eat from restaurants or street vendors? Knowing these kinds of things helps me narrow my advice to better suit your needs.
Hope this is helping point you in the right direction.
-Matt
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